Field of the Invention
The present invention relates to a radar apparatus which can measure a distance to an object and which can determine whether or not the object is a pedestrian. Notably, the term “pedestrian” used herein encompasses not only a pedestrian but also a person in an automobile or the like.
Background Art
A radar apparatus is known as an apparatus for detecting an object. The radar apparatus radiates a radio wave toward an object, receives a reflection wave from the object, and detects the distance to the object and the velocity and bearing of the object. Examples of known methods for measurement of distance and velocity include a FM-CW method, a multi-frequency CW method, and a pulse method. Examples of known methods for measurement of bearing include a digital beam forming method and a phase mono-pulse method. In recent years, radar apparatuses are mounted on automobiles, etc., and are used for controlling the distance between vehicles and avoiding collision.
Patent Document 1 discloses a method of determining whether or not an object detected by a radar apparatus is a pedestrian. According to the method disclosed in Patent Document 1, the distance to an object and the bearing of the object are detected by a radar apparatus, image data are obtained by a camera, and the determination as to whether or not the object is a pedestrian is made by analyzing image data of a region determined by the distance and bearing of the object detected by the radar apparatus.
Patent Document 3 discloses a method of determining the type of an object as follows. Of a plurality of peaks of a frequency spectrum obtained by a radar apparatus, a certain peak is compared with a corresponding peak measured the last time so as to calculate the amount of change with time (hereinafter referred to as the “time cause change”). The amount of change is compared with a probability distribution table, and the type of an object which is most likely to have such a change amount is determined to be the type of the actual object. Similar determination is made for other peaks. For example, a vehicle is small in the time cause change of a peak, and a pedestrian is large in the time cause change of a peak. Therefore, determination as to whether an object is a vehicle or a pedestrian can be made by the above-described method.
In the case where a radar apparatus is used to measure the distance between the apparatus and an object, since the object is generally a three-dimensional object having a certain size, a plurality of reflection points may be present on the object, and consequently, a plurality of peaks may be detected. Namely, even though only one object is present, the radar apparatus may erroneously determine that a plurality of objects is present. Therefore, it is necessary to group the detected peaks on an object-by-object basis such that the number of groups coincides with the number of objects, to thereby allow correct detection of the objects.
Patent Document 2 discloses a method of grouping peaks. Patent Document 2 describes that a plurality of peaks which differ in bearing angle and are the same or approximately the same in distance and which form a local maximum are grouped into a single group. Patent Document 2 also describes that the bearing angle of the object is detected through image recognition by a camera or the like, and the grouping is corrected. For example, in the case where peaks associated with two objects adjacent to each other are detected such that they belong to a single group, the group of peaks is divided into two groups by the correction performed through image recognition.    Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2009-295184    Patent Document 2: Japanese Patent Application Laid-Open (kokai) No. 2010-151621    Patent Document 3: Japanese Patent Application Laid-Open (kokai) No. 2012-112653
However, the determination method of Patent Document 1 increases cost, because the method requires a camera in addition to a radar apparatus. Therefore, there has been demand for a method of determining whether or not an object is a pedestrian, at lower cost.
The pedestrian determination method of Patent Document 3 has the following problem. For example, in a situation where a vehicle is present behind a pedestrian, a reflection wave from the vehicle is blocked by the pedestrian, and the peak value changes greatly. If the pedestrian determination method of Patent Document 3 is performed on the basis of such a change amount of the peak value, the vehicle is also highly likely to be determined to be a pedestrian. Therefore, in the case where the pedestrian determination method of Patent Document 3 is used for applications such as an automatic drive system and a collision prevention system of a vehicle, the above-mentioned great change of the peak value causes an erroneous operation.